The present invention relates to a method and apparatus for forming a web having a predetermined grammage profile in its transverse direction.
When forming a web of particulate material such as wood or synthetic fibers, it is known to use air to transport the particles in a forming head to an airpermeable wire on which the particulate material is collected and forms a web while the air passes through the web and the wire and is drawn off via a suction box by means of a slight subatmospheric pressure generated by a fan system. The wire is driven forward at a controlled speed so that a web having a certain weight per unit area is formed. In order to reduce variations in the grammage (i.e. weight per unit area) which may occur during forming, at least in the manufacture of thicker webs, a scalper roll is used which cuts excess material from the upper side of the web. The position of the scalper roll in relation to the wire can be controlled by measuring equipment located after the roll for measuring grammage. This equipment may comprise a weighing plate or some other type of grammage meter. This procedure enables a web to be produced with uniform grammage in its longitudinal direction.
Substantially uniform grammage is also desired in the transverse direction of the web. In certain cases it is even desirable to be able to vary the grammage across the web, the edge portions of the web having greater grammage than the central portion, for instance, since experience has shown that a certain squeeze effect will occur at the edge portions of the web during subsequent treatment of the web. In the manufacture of fiberboard, for instance, air is squeezed out from the edge portions of the subsequent belt pre-compression and hot compression steps. If the web has been formed with suitably increased grammage in the edge portions, the final result will then be that the finally pressed board will be substantially uniform in grammage and density transverse to the direction of forming, which is important if it is to be acceptably strong at the edge portions. A web with initially uniform grammage in its cross direction, though, will have lower grammage and density at the edge portions in the final board. The edge portions of the board will therefore have low strength properties. The properties of the edge portions will determine if the product is to be classed as first or second quality. Therefore, the choice is between increasing the average grammage in order to obtain acceptable properties in the edge portions, or sawing off the unacceptable part of the edge portions. Both alternatives result in extra material consumption and increased manufacturing costs.
To control the grammage across the web, it is known when using air carried fibers to give the fiber flow entering the distribution chamber of the forming head an oscillating movement transverse to the direction of movement of the web. This oscillation can be achieved either mechanically as is described in U.S. Pat. No. 3,071,822 or pneumatically as described in U.S. Pat. No. 4,099,296 (substantially corresponding to SE No. 7510795-3).
The distribution of fibers across the web in apparatuses using pneumatically controlled fiber distribution has not been satisfactory in that it has been necessary in practice to apply rolls or loaded sliding shoes to press down the edge portions of the fiber web in an attempt to achieve increased grammage at the edge portions. The use of rolls or sliding shoes has considerable drawbacks. For one thing, the load distribution must be varied for varying grammage in order to achieve an acceptable result; and for another, there is a considerable risk that the upper surface of the fiber web will be rolled up, roughened or otherwise destroyed. Apparatuses using mechanically controlled fiber distribution such as the apparatus described in the aforementioned patent do not succeed in achieving the desired grammage profile across the web and there, too, it has been necessary in practice to use rolls or sliding shoes similar to those described above, in order to improve the result.
In other applications it has been necessary to camber the scalper roll to a certain extent in order to at least improve the grammage profile across the width of the web. This has the obvious drawback that the desired grammage profile across the width of the web can only be obtained at nominal grammage.
The object of the invention is to minimize the problems mentioned above and to provide a method and an apparatus for forming a web in such a manner and using such means that a predetermined grammage profile can be continuously obtained, so that desired variations in grammage across the web can be controlled and adjusted automatically in a reliable manner.